In the oil and gas industry, a subterranean formation (i.e. a “reservoir”) is often treated (or “stimulated”) to enhance or restore the productivity of a well. Typically, a large number of well related vehicles and equipment are used at a well site during a treatment operation. Stimulation treatment operations may include, for example, blending units, pump units, manifold trailers, acid injection units, proppant transport units, and other types of equipment for numerous potential procedures. Typically, each type of equipment or unit is mounted on its own vehicle and trailer, or set of vehicles and trailers, and operated by a crew dedicated to that particular type of equipment.
Preparation of the area around the wellhead often is dictated by the number and size of equipment desired for a given project. Each vehicle type and corresponding crew should have sufficient room at the well site to access the well during its specific procedure. Downtime can occur between some operations while waiting for the arrival of crews to handle specific procedures in a desired sequence during the oilfield operation.
In hydraulic fracturing, fracturing fluid is injected into a wellbore, penetrating a subterranean formation and forcing the fracturing fluid at pressure to crack and fracture the strata or rock. Proppant is placed in the fracturing fluid and thereby placed within the fracture to form a proppant pack to prevent the fracture from closing when pressure is released, providing improved flow of recoverable fluids, i.e., oil, gas, or water. The success of a hydraulic fracturing treatment is related to the fracture conductivity which is the ability of fluids to flow from the formation through the proppant pack. In other words, the proppant pack or matrix may have a high permeability relative to the formation for fluid to flow with low resistance to the wellbore. Permeability of the proppant matrix may be increased through distribution of proppant and non-proppant materials within the fracture to increase porosity within the fracture.
Prior to injection of the fracturing fluid, the proppant and other components of the fracturing fluid may be blended. Hydraulic fracturing operations may blend and pump more than 3 million pounds or 1.3 million kilograms of proppant or dry components per day at a wellsite. Proppant is often stored in silos or other types of units on site, which deliver the proppant into a hopper associated with a blending unit. The proppant is then metered from the hopper into a mixer.
Dry components, such as proppants, and liquid components, such as gels, may be blended into the fracturing fluid, often referred to as a slurry, in a blender. Blenders, such as the blender described in U.S. Pat. No. 4,453,829, may have slinger elements of a toroidal configuration with a concave upper surface. Several upstanding blade members are mounted on the concave surface of this slinger and an impeller member is attached to the underside of the slinger. The slinger and impeller are enclosed within a housing and fastened to the end of a drive shaft rotated by a motor mounted above the housing. A hopper is mounted above an inlet eye in the top of the housing, for introducing sand or other solid particles or dry components into the housing. At the bottom of the housing is a suction eye inlet, for drawing fluid or liquid components into the housing and the resulting fluid-solid mixture is discharged through an outlet port in the housing.
In the operation of the blender described above, sand flows out of the hopper and drops onto the rotating slinger through the inlet eye in the housing. With the impeller and slinger rotating at the same speed, the vortex action of the impeller creates a suction force that draws liquid into the casing through the suction eye inlet. As the liquid is pulled into the casing it is pressurized by the impeller and mixed thoroughly with the sand being flung outwardly, in a centrifugal action, from the slinger. The sand-liquid mixture is then continuously discharged, under pressure, through the outlet port, from which it is carried into the pump unit and injected into a well. Some blenders, such as the one described above, may cause air within the dry component to become entrained in the slurry.
Other blenders, such as the one described in U.S. Pat. No. 4,614,435, are designed to mix dry components with fluid components without entraining air into the resulting slurry. The dry components are contained in a hopper mounted above the inlet eye of a housing member. The outlet end of the hopper sets above the inlet eye to provide an exterior air exhaust space at this point on the blender. The housing encloses a slinger and impeller member that is fastened to the underside surface of the slinger.
The impeller and slinger are both fastened to the bottom end of a drive shaft that extends up through the inlet eye of the housing to a motor that rotates the shaft. The slinger has a toroidal configuration and a topside concave surface that faces toward the top of the housing. The underside surface of the slinger has a recess in it and the recess defines an interior air exhaust space between the slinger and impeller. The slinger also has one or more interior air exhaust channels that extend from the air exhaust space between the slinger and impeller up to the topside surface of the slinger. To obtain a desired pressure output of 60 to 80 PSI (Pounds per Square Inch), the slinger and impeller may be rotated at a speed between 1,200 and 1,400 RPM (Revolutions per Minute). The high rotational speed in conjunction with the abrasive nature of the proppant being agitated by the impeller and slinger causes erosion on the impeller and slinger components and often causes the blender to wear out, necessitating frequent maintenance and rebuilding.
In addition to the above mentioned blenders that provide a pressurized output above hydrostatic pressure, tub blenders are also used. Tub blenders separate the mixing and pumping operations. A tub mixer delivers proppant and fluid into a large tub which contains an agitation mechanism, such as a rotational paddle or horizontal ribbon mixer. Mixing of the dry component and liquid component occurs in this tub at hydrostatic pressure due to gravity, and a centrifugal pump then takes fluid from the bottom of the tub and discharges the fluid under pressure at about 80 PSI to high pressure fracturing pumps or a manifold trailer connected to the pumps.
Further, some blenders use centrifugal pumps to pump clean liquid components into a closed tub with a rotating slinger at the top of the tub. The centrifugal pump pressurizes the entire tub, and the slinger introduces and mixes the dry component into the liquid component to create the slurry. The slurry then exits the tub at a tangential discharge point in the housing. The slinger within the tub does not impart energy to the slurry above the energy received from the centrifugal pump as a result of the centrifugal pump pressurizing the tub.
In any type of blender used for creating the slurry, there are components that undergo erosion and wear due to the highly abrasive nature of the proppant within the slurry. Additionally, some blenders may also present issues with respect to maintaining sufficient discharge pressure to the high pressure pumps or manifold. The high pressure pumps may be located on the wellsite at a considerable distance from the blender unit, at times being over 150 ft or over 45 m away from the blender. The pressure drop through the hose extending between the blender unit and the high pressure pump or manifold may cause insufficient suction pressure conditions at the high pressure pumps thereby causing undue wear on the high pressure pumps due to starvation or cavitation.
Blenders are typically employed to mix components of a fracturing fluid together in a single blender. Fiber products have traditionally been difficult to handle and meter at the desired concentrations in both stimulation and cementing work. Reliability problems that typically arise with the existing fiber metering and delivery systems include the fiber jamming the metering equipment and plugging conveyance chutes. Thus, a separate fiber-to-liquid component interface is desirable that prevents plugging and is not subject to the restrictive geometry of current fiber chutes.